1. Field of the Invention
The present invention relates to the variable speed control of electric motors. More particularly, the present invention relates to the utilization of a single speed controller applied to a family of multi-tap electric motors.
2. Discussion of the Related Art
Many applications for electric motors demand variable speeds with a known load on the motor. For example a blower motor in a household heating, ventilation and air-conditioning (HVAC) system will typically be a fractional horsepower motor driving a blower unit or fan blade which represents a known load varying regularly by speed in revolutions per minute.
Multi-tap motors may desirably be utilized in many applications. These motors, while attaining a plurality of fixed selectable speeds due to the multiple operating taps by mechanical switching between the taps, have not been particularly well adapted for continuously variable speed usage. Rather they are designed to operate efficiently only at one best speed within a tap and inefficiencies may result when trying to run the motor at other than the designed speed. However, many systems, such as the above HVAC applications, would benefit greatly from having a wider range of motor speeds available.
Also in the past art, a variable range of speeds from one induction motor was obtained through the use of expensive controllers changing the frequency and voltage of the input to the motor windings, throughout the operating range of the induction motor. Expensive controllers such as these were necessary because, as the input to the motor windings strays farther from sinusoidal, motor efficiency and power factor drop while total harmonic distortion rises, resulting in unacceptable noise, heat, efficiency loss, and motor life. Further, known motor controllers utilizing inexpensive switching mechanisms, such as triacs, to control power to the motor windings by “chopping” the sinusoidal waveform input were thought to be of limited use in applications of continuously variable motor speed control.
Further, the development and utilization of multiple control algorithms and control board apparatuses for each individual motor, or motor application, within a generic application such as an HVAC system, results in increased expense for the manufacturer and consumer of such systems and may further complicate maintenance and repair of such systems.
It would therefore be desirable to obtain an efficient variable speed operation of multi-tap motors for HVAC systems. It would also be desirable to obtain such control over a family of such multi-tap motors or motor applications through the use of a single controller system.